The present invention relates to a method for manufacturing an aluminum nitride (AlN) single crystal useful as a dispersant (filler) for improving mechanical strength and heat radiation ability, or as a substrate material, heat radiating material, or structural material for parts for electronics or electrical machinery and appliances, particularly as a material for applications having a high heating value such as a semi-conductive laser element and a light-emitting diode.
In apparatuses and devices using parts having a high heating values, such as parts for electronics or electrical machinery and appliances, optical apparatuses, or OA apparatuses, quick radiation of heat generated is required. Therefore, as a substrate material and a heat radiation material (heat sink), which are brought into contact with these parts, or as a structural material (hereinbelow referred to as a “substrate material or the like”) for these parts, there has conventionally been employed metallic material having high thermal conductivity and excellent heat radiation ability (e.g., aluminum (Al) and cupper (Cu)).
However, in recent years, apparatuses and devices using such parts have tended to be minimized and densified and to raise output. The level of heat radiation ability required to a substrate material or the like has become higher. In addition, in some cases, properties such as mechanical strength and electric insulation, which cannot be imparted sufficiently to a metallic material, are required. In view of such a background, aluminum nitride is now employed as the substrate material or the like, which is excellent in various properties such as mechanical strength, thermal resistance, corrosion resistance, and electric insulation in addition to heat radiation ability.
It is generally known that a sintered body of aluminum nitride is used as a structural material for a substrate material or the like. However, attention has recently been paid to an aluminum nitride single crystal (bulk single crystal or whisker), which may be able to constitute a substrate material or the like having higher performance. A bulk single crystal is expected to be used as a substrate material or the like by the use of heat radiation ability thereof similarly to a sintered body, as a semi-conductive laser element or a light-emitting diode because of its wide energy-band gap (6.2 eV), and as a substrate material for a semi-conductive laser element or a light-emitting diode because it has the same extent of lattice constant and thermal expansion coefficient as those of gallium nitride (GaN). Meanwhile, a whisker is also expected to be used as a dispersant (filler) for improving mechanical strength and heat radiation ability of metals and plastics because of its excellent mechanical strength and heat radiation ability.
As methods for manufacturing an aluminum nitride single crystal, various methods such as nitriding, fluxing, chemical transportation, sublimation, and chemical vapor phase synthesis have been known. However, since aluminum nitride is stable against heat, it hardly melts even under the condition of high temperature, and it is very difficult to grow a large crystal. Therefore, very few examples of manufacturing an aluminum nitride single crystal with a sufficient size for practical use as a substrate material or the like have been reported.
Some of the very few examples will be shown. For example, there has been reported a method for manufacturing a nitride single crystal, the method being characterized by mixing oxide powder, which reacts with the nitride under heating to decompose and gasify nitride, with a nitride powder to obtain a mixed powder; heating the mixed powder at a temperature lower than the sublimation temperature or melt temperature of the nitride in a nitrogen atmosphere or the like; and subjecting the decomposed gasified component to crystal growth on a substrate from a vapor phase (see JP-A-10-53495). According to this method, it is considered that an aluminum nitride single crystal with dimensions of 10 mm×10 mm or more and a thickness of 300 μm or more, which is sufficiently large as a bulk material, can be obtained.
There has also been reported a method for manufacturing an aluminum nitride bulk single crystal, characterized by brining nitrogen into contact with a molten metal aluminum to form aluminum nitride in the molten body, and allowing the aluminum nitride to accumulate on a seed crystal which is in physical contact with the molten body (see JP-A-2003-505331). According to this method, it is considered that an aluminum nitride crystal having a diameter of 1 inch (about 2.5 cm) or more can be obtained.
However, there is still room for improvement from the viewpoint of productivity, since it takes a long time by using the above methods to grow an aluminum nitride single crystal sufficiently large for practical use as a dispersant for improving mechanical strength and heat radiation ability or as a substrate material or the like of parts for electronics or electrical machinery and appliances.
To be concrete, it is considered that the method described in JP-A-10-53495 has a low crystal-growth speed because of the long hold time of about 24 hours at the reaction temperature. Therefore, this method cannot give satisfactory crystal-growth speed considering practical use on an industrial level, and it therefore has a problem from the viewpoint of productivity and cost. Though the method described in JP-A-2003-505331 can achieve a relatively high crystal-growth speed of about 1.6 mm/hour, the method requires an expensive special device provided with complex mechanisms such as a nitrogen gas injector, a crystal raiser, and various controllers. Since this method cannot employ a widely used device and lacks wide usability, it is not necessarily suitable for practical use on an industrial level. That is, a method for manufacturing an aluminum nitride single crystal has never been disclosed as being capable of obtaining an aluminum nitride single crystal which is sufficiently large for practical use at low cost in a short time and having high productivity and wide usability, and such a method is earnestly desired by the industrial world.